Polarity of Organic Solvent/Water Mixtures Measured with Reichardt’s B30 and Related Solvatochromic Probes – a Critical Review

Abstract

The UV/Vis absorption energies (νmax) of different solvatochromic probes measured in co-solvent/water mixtures are analysed as a function of the average molar concentration (Nav) of the solvent composition. Nav is discussed in terms of the underlying average molar mass (mol average or mass average) of the mixture. ET(30) of Reichardt`s dye B30 is the focus of the analysis. Methanol, ethanol, 1,2-ethanediol, 2-propanol, 2-methyl-2-propanol, 2-butoxyethanol, formamide, N-methylformamide (NMF), dimethylformamide (DMF), N-formylmorpholine (NFM), 1,4-dioxane and DMSO were used as co-solvents. Significant linear dependencies of ET(30) as a function of Nav can be demonstrated for formamide/water, 1,2-ethanediol/water, NMF/water and DMSO/water mixtures over the whole composition range. These mixtures belong to the group of solvents that do not enhance the water structure according to the Marcus classification. The specifics of the 1,4-dioxane/water mixtures are discussed. For alcohol/water mixtures with enhanced water structure, the influence of the solvent microstructure on ET(30) is particularly evident.